Programming method for non-volatile memory

ABSTRACT

In an embodiment, a phase change non-volatile memory includes a number of memory cells. The memory cells include a phase change material which may transition between two memory states. The phase change material has different electrical properties in different states. The memory cells may be electrically addressable and include a transistor in each cell to electrically read and write data to the cell. An energy beam may be used to pre-program the device by heating selected memory cells, and consequently changing the state of the phase change material.

BACKGROUND

[0001] Certain non-volatile memory devices utilize phase changetechnology to read and write data. The storage mechanism is typically areversible change of state of a material or structure. For example,CD-Rewritable (CD-RW) and DVD-RAM optical disk drives use laser-inducedstructural phase change in an alloy layer on the disk to read and writedata. The disk drives use laser energy to heat the material betweenamorphous and crystalline states to write data, and use the differencein reflectivity between the two states to optically read data.

[0002] Another class of phase change non-volatile memory devices utilizethe electrical properties of the phase change material to read and writedata, taking advantage of the difference in resistivity in the materialin the different states. In the amorphous state, a small amount ofcurrent will pass, and in the crystalline state, the resistance of thematerial in that state will limit the current. Such devices may use anelectric current to heat the material between amorphous and crystallinestates.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a perspective view of a memory device according to anembodiment.

[0004]FIG. 2 is a flowchart describing a preprogramming method accordingto an embodiment.

[0005]FIG. 3 is a plan view of a memory device and illustrates ascanning pattern for preprogramming the memory device according to anembodiment.

[0006]FIG. 4 is a block diagram of a memory device according to anembodiment.

DETAILED DESCRIPTION

[0007] An optically preprogrammable electrically-addressable memorydevice 100 according to an embodiment is shown in FIG. 1. The memorydevice may be a non-volatile memory, which includes an array ofindividually addressable memory cells 102 arranged in rows and columns.Each memory cell includes a phase change material that has differentelectrical properties in different states, e.g., amorphous andcrystalline states. The electrical properties may include resistivity.Individual cells may represent different bits of stored information.Such an electrically addressable non-volatile phase change memory devicemay be used as direct replacements for other types of non-volatilememories such as Flash memories and volatile memories such as DRAMs.

[0008] In normal operation, a circuit 104 may be used to write, erase,and read information stored in the memory cells 102. The circuit 104 maybe used to address individual memory cells 102 and to provide electricalenergies used to change the state of the material and to read the datain the cell. A technique is provided for preprogramming cells in thearray using an external energy source, such as an optical element, e.g.,a scanning laser.

[0009] A non-volatile memory device may utilize the electricalproperties of phase change materials to store data. For example, thememory device 100 shown in FIG. 1 may utilize a chalcogenide alloy,similar to that used in many CD-RW and DVD-RAM optical disks, as a phasechange material. The circuit 104 may provide electrical energy toconvert a small volume of the phase change material in one or moreselected memory cells to a crystalline or to an amorphous state, whichmay be read as a “0” or “1” value, respectively. The phase conversionmay be accomplished by heating the material. However, unlike in therewritable optical disks described above (CD-RW and DVD-RAM), thecircuit 104 provides the energy used to heat the material instead of alaser beam.

[0010] Heating the material in the crystalline phase above its meltingpoint causes the material to lose its crystalline structure. When thematerial then cools below the glass transition temperature, the materialis locked in its amorphous phase. The amorphous phase may be stable atroom temperature, but the rate of nucleation and growth of crystallitesmay increase rapidly as the temperature of the material is raised towardthe melting temperature. To switch the memory element back to thecrystalline phase, the circuit 104 heats the material between the glasstransition temperature and the melting temperature, causing rapidnucleation and crystal growth.

[0011] A memory cell may be read by applying an electric field to thecell. The material has a lower resistance in the crystalline state, anda small current will pass. The material has a relatively higherresistance in the amorphous state, and the applied voltage and theresistance of the material will limit the current through the cell.

[0012] It may be desirable to preprogram the memory device during themanufacturing process, before it is packaged and installed in a largerdevice. When the array is preprogrammed by electrical means, i.e., atransistor array, the current used to write to the memory cells may belimited by the constraints of the structure. This may increaseprogramming time.

[0013]FIG. 2 is a flowchart describing a preprogramming method 200 usingan external energy source. The external energy source, in this example,a scanning laser beam, may provide more energy than the electricalprogramming method, and hence, may enable faster programming.

[0014] A scanning laser controller 106 may scan a laser beam 108 overindividual memory cells 102 in the array. An exemplary scanning pattern300 is shown in FIG. 3. The laser scans one row of the array in the Xdirection and then moves slightly in the Y direction before scanningaccess another row in the focal plan in the X direction.

[0015] The material in the memory cells 102 may originally be in thecrystalline phase. Depending on the bit sequence loaded into thescanning laser controller for preprogramming, the laser may selectivelyheat memory cells corresponding to a “0” value above the melting point,causing a transition to the amorphous and less resistive state (block204). The laser may scan the entire array of a portion of the array tobe preprogrammed (block 206). Once preprogrammed, the memory device maybe packaged and installed in a larger system or device (block 208).

[0016] After the initial preprogramming by the external energy source,the memory device may be read and written, including furtherprogramming, electrically using the circuit 104 (block 210). Oncepackaged and installed, the memory array may be sealed, and may not befurther accessible by the laser.

[0017] Depending on the size of the memory cells, a tightly focusedlaser beam may be required to produce a very small spot on the surfaceof the array for scanning the individual memory cells 102. The size ofthe spot may be controlled by selecting one or more optical lenses forfocusing the laser beam. For example, high numerical aperture (NA)lenses and solid immersion lenses (SILs) may produce such highly focusedlaser beams. High NA lenses are used for high resolution opticallithography and both types are used in optical and magneto-optical (MO)disk drives for reading and writing data.

[0018] The preprogrammed data may include identification informationidentifying the memory device. The preprogrammed data may also includeparameters that identify the device to the system in which it isinstalled. This information may indicate the memory device's type andcapacity.

[0019] The memory device 100 may be operable in different devices. Thepreprogrammed data may include one or more selectable configurations thememory device 100 may operate in. A user may select the configurationappropriate for the system in which the memory device will be used.

[0020] The preprogrammed data may include instructions and/or dataapplicable to a particular application. This information may bere-writable, e.g., useful for set up on first use, or may be permanent,e.g., read only. The memory device 100 may include structure forprogramming the memory elements in the array, as shown in FIG. 4. In thecase of permanent preprogrammed data, some of this programming structure401 may be eliminated for the sector of memory that is read only, whileremaining electrically addressable by the circuit 104 for accessing theprogrammed instructions and/or data. This may reduce the overall area ofthe device.

[0021] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. For example,blocks in the flowchart may be skipped or performed out of order andstill produce desirable results. Accordingly, other embodiments arewithin the scope of the following claims.

1. A method comprising: optically pre-programming anelectronically-programmable phase change memory device.
 2. The method ofclaim 1, wherein the memory device comprises a non-volatile memory. 3.The method of claim 1, wherein the memory device includes a plurality ofmemory cell, each cell including a phase change material.
 4. The methodof claim 3, wherein said optically pre-programming comprises heating oneor more of said cells with an energy beam; and changing a memory stateof the heated cells.
 5. The method of claim 4, wherein the energy beamcomprises a laser beam.
 6. The method of claim 1, further comprisingreading data from said memory device electronically.
 7. The method ofclaim 6, wherein said reading data comprises determining an electricalcharacteristic of the phase change material in memory cells in thememory device.
 8. The method of claim 7, wherein the electricalcharacteristic is resistance.
 9. The method of claim 1, furthercomprising writing data from said memory device electronically.
 10. Themethod of claim 8, wherein said writing comprises heating memory cellsin the memory device using an electrical current.
 11. The method ofclaim 1, wherein said pre-programming comprises writing deviceidentification information to the memory device.
 12. The method of claim1, wherein said pre-programming comprises writing configurationinformation to the memory device.
 13. The method of claim 1, whereinsaid pre-programming comprises writing data to the memory device. 14.Apparatus comprising: an energy source operative to generate anoptically focused energy beam; and a stage for holding anelectronically-programmable phase change memory device; and a controlleroperative to cause the energy beam to scan a pattern over the memorydevice, thereby pre-programming said memory device.
 15. The apparatus ofclaim 14, wherein the memory device comprises a non-volatile memory. 16.The apparatus of claim 14, wherein the memory device includes aplurality of memory cell, each cell including a phase change material.17. The apparatus of claim 16, wherein the controller is operative to:heat one or more of said cells with an energy beam; and change a memorystate of the heated cells.
 18. The apparatus of claim 17, wherein theenergy beam comprises a laser beam.
 19. The apparatus of claim 14,wherein the controller is operative to read data from said memory deviceelectronically.
 20. The apparatus of claim 19, wherein the controller isoperative to read data by determining an electrical characteristic ofthe phase change material in memory cells in the memory device.
 21. Theapparatus of claim 20, wherein the electrical characteristic isresistance.
 22. An article comprising a machine-readable mediumincluding machine-executable instructions, the instructions operative tocause a machine to: optically pre-program an electronically-programmablephase change memory device.
 23. The article of claim 22, wherein thememory device comprises a non-volatile memory.
 24. The article of claim22, wherein the memory device includes a plurality of memory cell, eachcell including a phase change material.
 25. The article of claim 24,wherein the instructions for optically pre-programming includeinstructions operative to cause the machine to: heat one or more of saidcells with an energy beam; and change a memory state of the heatedcells.
 26. The article of claim 25, wherein the energy beam comprises alaser beam.
 27. The article of claim 22, further comprising instructionsoperative to cause the machine to read data from said memory deviceelectronically.
 28. The article of claim 27, wherein the instructionsfor reading data include instructions operative to cause the machine todetermine an electrical characteristic of the phase change material inmemory cells in the memory device.
 29. The article of claim 28, whereinthe electrical characteristic is resistance.